This invention relates to surfactant systems for recovery of oil from subterranean reservoirs.
It has long been known that the primary recovery of oil from a subterranean formation leaves a substantial amount of the oil in the formation. This has led to what is commonly referred to as secondary recovery or water flooding where a fluid such as brine is injected into a well to force the oil from the pores of the reservoir toward a recovery well. However, this technique also leaves substantial amounts of oil in the reservoir, so-called residual oil, because of the capillary retention of the oil. Accordingly, surfactant systems have employed either in place of the secondary recovery or more generally in a tertiary recovery process. One particularly suitable type of surfactant system is that which results in the in situ formation of a microemulsion which is immiscible with the oil it is deplacing. Such microemulsion systems are very effective in removing residual oil.
The surfactant systems employed to produce microemulsion type oil recovery basically contain at least three separate ingredients, brine, a surfactant, and a cosurfactant. It is disclosed in Hedges et al, U.S. Pat. No. 4,265,308 (May 5, 1981) how to correlate these three ingredients so as to arrive at a combination of surfactant and cosurfactant at a unique salinity concentration at which maximum oil recovery is obtained.
Work with surfactant systems to produce model compositions is conventionally done using sodium chloride as the salt in the brine. However, many reservoirs in fact contain a mixture of salts in the brine and different salts cause a different interrelationship between the necessary combination of surfactant and cosurfactant required to give good oil recovery.